a) Field of the Invention
The present invention relates to a method for continuously preparing an amino resin particulate having narrow particle size distribution.
The amino resin particulate having narrow particle size distribution can be used as fillers of rubber and plastics, organic pigments for coating compounds and as fillers of papers for improving printability and optical properties such as opacity and brightness of paper.
In particular, in the case of using the amino resin particulate having narrow particle size destribution as a filler of thin papers, one pass retention in the papers becomes superior compared with inorganic fillers and essential performance of the resin particle can be maintained without further pulverization to finer mesh even in a super calender treatment conducted in the final processing step of paper making.
b) Description of the Prior Art
It has been well known that condensation reaction of amino compounds and aldehydes is carried out in the presence of an acid catalyst to form a solid product and the solid product is then crushed and classified to prepare aggregates of particulates.
Japanese Patent Publication No. 13491 (1976) discloses that ureas or phenols and formaldehyde, or prepolymers derived from these raw materials are reacted in the presence of a protective colloid and an acid catalyst to obtain an aggregate having a particle size of 1 to 50 .mu.m. The aggregate can be used in order to provide various color developers of resin with desired flatting efficiency and tone.
Japanese Patent Publication No. 7476/1978 describes a method for preparing cured amino resin particulates by the formation of particles using the reaction of melamine or ureas with aldehydes. It is also disclosed that the average particle size of the cured resin is 10 .mu.m or less and the resin particulates can be used for an organic pigment as such or after coloring.
Japanese Patent Laid-Open No. 115921/1986 teaches that a solid product of a crosslinked urea-formaldehyde resin is converted to a slurry and then milled with an impact grinder to obtain particles useful for fillers of papers. In this, particles of the crosslinked urea-formaldehyde polymer, each particle having an average particle size of 0.1 to 1.0 .mu.m (hereinafter referred to as a primary particle) aggregates to form a particle having an average particle size of 1 to 30 .mu.m (hereinafter referred to as a secondary particle). The quality of paper products can be improved by the addition of the secondary particle to papers.
In the above prior arts, merely the average particle size of the secondary particle is described on the cured resin in many cases. For example, a nominal average particle size of 10 .mu.m has often contained large particles having maximum diameter of more than 60 .mu.m or a fairly large portion of small particles having a diameter of 3 .mu.m or less. Thus, considerable variation has been found on the particle size distribution depending upon the production conditions of the particles.
Japanese Patent Laid-Open No. 159596/1988 describes that a crosslinked urea-formaldehyde polymer having a specific particle size distribution of the secondary particle exhibits excellent performance toward paper products as compared with conventional polymers having a broader particle size distribution.
However, it has been desired to establish a production method of polymers having further preferred particle size distribution.